The present invention relates to an improvement in a device for detecting secondary electrons from a specimen in a scanning electron microscope.
Recent transmission type electron microscopes are equipped with an additional device allowing observation of the scanning microscope image with secondary electrons. With such a modified scanning electron microscope, it is customary for the specimen to remain inserted in a gap between the magnetic pole pieces of an objective lens when observing the scanning image, as it was when observing a transmitted microscope image. An example of such a combination of prior art is illustrated in FIG. 1, in which designated at 1 is a system for illuminating a specimen with an electron beam, the system comprising an electron gun for producing the primary electron beam along an optical axis 2 and a condenser lens for converging the electron beam. The specimen 4 is inserted substantially centrally in a gap between the magnetic pole pieces of an objective lens 3. The electron beam for illuminating the specimen along the optical axis is focused on the specimen surface by a magnetic field generated in front of (toward the electron gun) the specimen. The objective lens magnetic field in front of the specimen thus acts as a final stage condenser lens, and also acts as the deflecting means together with a deflection coil 5 disposed above the upper magnetic pole piece of the object lens so that the electron beam scans two-dimensionally over a surface of the specimen, and further acts as the focusing means for focusing secondary electrons 6 emitted in all directions from the specimen surface toward the direction of the optical axis. A device for detecting the secondary electrons is disposed upwardly of the objective lens, and comprises a light pipe 7 with a scintillator attached to a front (toward optical axis) end thereof, a photomultiplier 8 positioned at the rear end of the light pipe, and others. The scintillator on the front end of the light pipe 7 is coated on its front (toward optical axis) face with a thin conductive layer. Such conductive layer and an accelerating ring electrode 9 therearound are held at a potential of the order of +10 KV by a d.c. power supply 10. A shield sleeve 11 which is at a ground potential is disposed in surrounding relation to the front end of the light pipe.
With the arrangement shown in FIG. 1, the secondary electrons emitted from the specimen 4 have relatively low energy ranging from several eV to several tens of eV, and hence become focused progressively in the direction of the optical axis 2 as the secondary electrons follow a spiral path. The secondary electrons tend to divert from the optical axis 2 again as they move out of the magnetic field formed by the objective magnetic pole pieces. The ring electrode 9, however, forms an electric field which extends above the objective lens and serves to accelerate the secondary electrons toward the scintillator. The secondary electrons as they hit the scintillator generate light, which is transmitted through the pipe 7 and converted by the photomultiplier 8 into electrical signals to be picked up. Since the primary electron beam which is focused onto the specimen has relatively high energy that is normally 20 KeV or higher, the degree to which the electron beam is deflected by the ring electrode 9 is negligibly small.